Polyhydroxyalkanoic acids are biodegradable and biocompatible, and their use for various molded products such as fibers or films has been studied.
A fiber produced from PHAs as a raw material is biodegradable and biocompatible, and thus, a great demand can be anticipated for the fiber as: medical equipment such as surgical sutures; fishery equipment such as fishing lines and fishing nets; clothing materials such as fibers; construction materials such as nonwoven fabrics and ropes; packaging materials for food or the like; etc.
Poly(3-hydroxybutanoic acid) (hereinafter, may also be referred to as “P(3HB)”) among PHAs is known to be synthesized by many microorganisms as an intracellular reserve substance and be accumulated in a form of granules in cytoplasm (Nonpatent Document 1).
Further, the inventors of the present invention have succeeded in obtaining P(3HB) with remarkably enhanced molecular weight using genetically modified Escherichia coli of a poly(3-hydroxybutanoic acid) synthesis gene compared to that obtained using a wild type P(3HB)-producing microorganism (Patent Document 1).
P(3HE) obtained from the P(3HB)-producing microorganism is expected to be a raw material for biodegradable products.
Fibers produced from P(3HB) as a raw material hitherto have been produced through a process involving: melt-extruding P(3HB) having a weight average molecular weight of about 600,000 (number average molecular weight of about 300,000) as a raw material; hot drawing the P(3HB); and subjecting the P(3HB) to heat treatment. A specific example of such a process described in Nonpatent Document 2 involves: purifying P(3HB) having a weight average molecular weight of 300,000 with chloroform; melt-extruding the P(3HB) in four stages of melting temperature zones (170° C.-175° C.-180° C.-182° C.); drawing the P(3HB) to a draw ratio of 800% at 110° C.; and maintaining the temperature at 155° C. for 1 hour to crystallize the P(3HB), to thereby form a fiber. Physical properties of the obtained fiber include a breaking strength of 190 MPa, an elongation to break of 54%, and a Young's modulus of 5.6 GPa. Further,  Nonpatent Document 3 describes a process involving: forming pellets having a viscosity average molecular weight of 360,000 once without purifying P(3HB) having a viscosity average molecular weight of 540,000; melt-extruding the pellets at 173° C.; winding at a wind rate of 2,000 to 3,500 m/min or 250 m/min; drawing to a draw ratio of 400% or 690% at 40 to 60° C.; and maintaining the temperature at 40 to 60° C. to crystallize, to thereby form a fiber. The physical properties of the obtained fiber include a breaking strength of 330 Mpa, an elongation to break of 37%, and a Young's modulus of 7.7 GPa.
However, the fibers do not have physical properties comparable to those of the general polymers and are not in practical use.
Meanwhile, Nonpatent Document 4 describes a process involving: melt-extruding non-purified P(3HB) granules at a melting temperature of 180° C. and a nozzle temperature of 170° C.; winding at a wind rate of 28 m/min; drawing to a draw ratio of 600% at 110° C.; and maintaining under tension of 0 MPa, 50 MPa, and 100 MPa at 75, 100, 125, and 150° C. for 2.5 minutes to crystallize, to thereby form a fiber. The obtained fiber has a breaking strength of 310 MPa, an elongation to break of 60%, and a Young's modulus of 3.8 GPa.
However, a fiber with high strength, a fiber with high strength and high modulus of elasticity produced from P(3HB) as a raw material having any molecular weight including purified P(3HB) and P(3HB) having a high weight average molecular weight of 600,000 or more, and a process for producing the same have not been found.
Thus, developments of processes for producing a fiber with high strength and a fiber with high strength and high modulus of elasticity having improved physical properties while retaining biodegradability from various PHAs as a raw material including PHAs of a wild type PHA-producing microorganism have been desired.
<Nonpatent Document 1>
Anderson, A. J. and Dawes, E. A, Microbiol. Rev. 54: 450-472 (1990)
<Nonpatent Document 2>
Gordeyev et al., J. Mater. Sci. Lett., 18, 1691 (1999)
<Nonpatent Document 3>
Schmack et al., J. Polym, Sci. Poylm. Phys. Ed., 38, 2841 (2000)
<Nonpatent Document 4>
Yamane et al., Polymer, 42, 3241 (2001)
<Patent Document 1>
JP 10-176070 A